The present invention relates to a closed-loop air fuel ratio control system and method for supplying an optimum air fuel mixture to an internal combustion engine.
Such a control system or method makes use of an exhaust gas sensor or oxygen sensor for sensing the concentration of the oxygen in the exhaust gas from the engine, and controls the air fuel ratio depending upon the result of the comparison between the output signal of the exhaust gas sensor and a predetermined reference value.
However, such a control system or method is arranged to only decide whether or not the sensor output signal is greater than the reference value, that is, whether or not the detected air fuel ratio is smaller than the stoichiometric value, and thus to control the air fuel ratio, without cylinder to cylinder variation, if it is applied to a multi-cylinder engine, to maintain the mean value among the cylinders constant. Accordingly, such control system and method are helpless against bad influences of a cylinder to cylinder air fuel ratio distribution on emission control and fuel economy.